The present invention relates in general to diode-laser printing. The invention relates in particular to diode-laser writing of data directly onto printing plates having a thermally sensitive organic coating for receiving the written data
Preparation of printing plates directly from computer generated data has been facilitated by the development of printing plate blanks comprising a metal base, typically aluminum, having a coating including a layer of a thermally-sensitive organic material. Such plates are often referred to by practitioners of the art as thermal plates. The organic material is formulated to undergo a change of state or conversion on exposure to thermal energy. This allows the computer generated data to be written directly into the layer of organic material. Printing ink adheres preferentially to the converted material. This direct writing avoids the process steps and chemicals required in printing plate preparation by photoresist exposure, development and etching. A semiconductor diode-laser provides a suitable thermal energy source. It is relatively small compared with other laser types, as well as being robust and reliable.
In one common computer-to-plate writing operation, a thermal plate is wrapped around a drum or cylinder. Data is written into the thermal plate, one line at a time, rotating the drum at the end of each line to expose an area of the plate on which to write the next line. Data is written on a line, one half-tone dot at a time, by exposing the organic material of the thermal plate to radiation emitted by a diode-laser. The diode-laser is switched on to begin writing a dot and switched off after a dot is written. Switching the diode-laser rapidly on and off for writing the data is accomplished by modulating current supplied to the diode-laser.
In computer-to-plate printers using a such a diode-laser, printing speed for any given thermally sensitive organic material is limited primarily by the amount of power emitted by the diode-laser. The faster the diode-laser is moved to speed up writing a line of data, the higher must be the modulation rate for data writing. The laser output-power must be correspondingly increased in order to maintain a minimum (threshold) thermal energy required for thermal conversion.
One possible approach to improving printing speed would be to use one or more linear-arrays of individually addressable diode-lasers (diode-laser bars). One problem, however, is that manufacturing tolerances in diode-laser bar preparation are typically insufficient to provide that each emitter in the bar has the same output power for a given input current. This could cause inconsistencies in the printed data. Further, individual diode-lasers (emitters) in such a linear array would need to be tightly packed to provide adequate quality of printed data, for example, spaced apart by about 200 micrometers (xcexcm). Each diode-laser can emit as much as 200 milliwatts (mW) of power in a single mode. Such tight packing and high output power of diode-lasers could cause optical and thermal cross-talk between diode-lasers such that the operation of one diode-laser can influence the operation of an adjacent one or more diode-lasers in the bar. This could exacerbate any manufacturing differences between diode-lasers in a bar, possibly even leading to data omission as a result of the output of an individual diode-laser falling below threshold energy for thermal conversion.
There is a need for a diode-laser bar arrangement which overcomes these problems, thereby facilitating the use of an array of individually addressable diode-lasers in writing data directly to a printing plate.
The present invention is directed to a printing head for writing data into a thermally sensitive medium. In one aspect, the invention comprises a diode-laser bar including a plurality of diode-lasers spaced-apart in a linear array. Each of the diode-lasers emits light in forward and reverse directions. A plurality of photodetectors is provided. The photodetectors are spaced apart in integrated linear array thereof corresponding in number and spacing to the plurality of diode-lasers. The photodetectors are arranged such that each thereof receives reverse-emitted light from a corresponding one of the diode-lasers. The inventive printing head further includes an application-specific integrated circuit (ASIC). The ASIC is arranged to individually monitor output of each of the photodetectors in response to the reverse-emitted light received thereby. Based on the monitored photodetector outputs, the ASIC supplies individual drive currents to each of the diode lasers such that all of the diode-lasers emit a predetermined target output power in the forward direction, the target power being the same for all of the diode-lasers.
In one preferred embodiment of the inventive printer head, the ASIC includes a plurality of sub-circuits. Each of the sub-circuits is in electronic communication with a corresponding one of the photodetectors for monitoring the output thereof, and with the corresponding diode-laser for supplying the individual drive current thereto. Each of the sub-circuits is supplied with a common potential, and each includes a reference current generator individually calibrated by calibration data derived from a particular light-output characteristic of the diode-laser and the target forward-emitted output power. Each of the sub-circuits includes a drive current generator for supplying the individual drive current to the diode-laser. Each of the sub-circuits is arranged to monitor current provided by the photodetector in response to reverse-emitted light received thereby from the diode-laser and to compare the monitored photodetector current with a reference current provided by the calibrated reference current generator. The drive current generator is adjusted, based on the current comparison, such that the individual drive current supplied thereby to the diode-laser causes the diode-laser to emit about the common target output-power in the forward direction.
In one example of the inventive printing head including an array of 50 single-mode diode-lasers, an array of 50 photodiodes (photodetectors) and an ASIC including 50 sub-circuits, all of the diode lasers emitted within xc2x11% of a nominal (target) output power of 150 mW.
While the present invention is described herein in the context of a printing head for a thermal printing application this should no be construed as limiting the invention. Those skilled in the art will recognize from the above-presented summary and the detailed description set forth below that the invention could find application in other fields, for example, in telecommunications and in diode-laser pumping of solid-state lasers.